Grazing incidence interferometers are generally known for determining the topography of a test surface, and among other advantages, they are easily adjustable in sensitivity to provide a zoom effect allowing a test surface to be measured with varying precision. They change sensitivity by adjusting the incidence angle; and they use a prism surface as a reference surface and place the test surface in contact with the reference surface.
Another development in interferometry separate from grazing incidence interferometers is phase modulation to move fringes so as to distinguish between hills and valleys on the test surface. This can be done manually with a finger tip touch by an observer who notices the fringe movement and determines the direction of test surface slopes. Phase modulation is also used in detection systems involving computer analysis to make the same distinctions between hills and valleys and use the computer to process and present information about the test surface. Phase modulation in computer controlled systems has been accomplished by piezoelectric transducers arranged to move the reference mirror.
To apply phase modulation as developed in the interferometry art to a grazing incidence interferometer would require relative movement between the test surface and reference surface that are normally placed in contact. Moving either the test surface or the reference surface relative to the other is so difficult and troublesome that phase modulation has not been applied to grazing incidence interferometers.
My invention recognizes a simple and effective way to accomplish phase modulation in a grazing incidence interferometer. It avoids the problems of moving the test or reference surfaces relative to each other and achieves excellent results in a simple and inexpensive way.